Glutamatergic and GABAergic neurons mediate much of the excitatory and inhibitory neurotransmission, respectively, in the vertebrate nervous system. The process by which developing neurons select between these two cell fates is poorly understood. Here we show that the homeobox genes Tlx3 and Tlx1 determine excitatory over inhibitory cell fates in the mouse dorsal spinal cord. First, we found that Tlx3 was required for specification of, and expressed in, glutamatergic neurons. Both generic and region-specific glutamatergic markers, including VGLUT2 and the AMPA receptor Gria2, were absent in Tlx mutant dorsal horn. Second, spinal GABAergic markers were derepressed in Tlx mutants, including Pax2 that is necessary for GABAergic differentiation, Gad1/2 and Viaat that regulate GABA synthesis and transport, and the kainate receptors Grik2/3. Third, ectopic expression of Tlx3 was sufficient to suppress GABAergic differentiation and induce formation of glutamatergic neurons. Finally, excess GABA-mediated inhibition caused dysfunction of central respiratory circuits in Tlx3 mutant mice.

Distinct classes of neurons are generated at defined times and positions during development of the nervous system. It remains elusive how specification of neuronal identity coordinates with acquisition of pan-neuronal properties. Here we show that basic helix-loop-helix (bHLH) transcription factors Olig2 and Neurogenin2 (Ngn2) play vital roles in the coordinated induction of pan-neuronal and subtype-specific properties of motoneurons. Olig2 and Ngn2 are specifically coexpressed in motoneuron progenitors. Misexpression studies in chick demonstrate the specific, combinatorial actions of Olig2 and Ngn2 in motoneuron generation. Our results further revealed crossregulatory interactions between bHLH and homeodomain transcription factors in the specification of motoneurons. We suggest that distinct classes of transcription factors collaborate to generate motoneurons in the ventral neural tube.

Most neurons in vertebrates make a developmental choice between two principal neurotransmitter phenotypes (glutamatergic versus GABAergic). Here we show that the homeobox gene Lbx1 determines a GABAergic cell fate in the dorsal spinal cord at early embryonic stages. In Lbx1-/- mice, the presumptive GABAergic neurons are transformed into glutamatergic cells. Furthermore, overexpression of Lbx1 in the chick spinal cord is sufficient to induce GABAergic differentiation. Paradoxically, Lbx1 is also expressed in glutamatergic neurons. We previously reported that the homeobox genes Tlx1 and Tlx3 determine glutamatergic cell fate. Here we show that impaired glutamatergic differentiation, observed in Tlx3-/- mice, is restored in Tlx3-/-Lbx1-/- mice. These genetic studies suggest that Lbx1 expression defines a basal GABAergic differentiation state, and Tlx3 acts to antagonize Lbx1 to promote glutamatergic differentiation.

Sensory information from the periphery is integrated and transduced by excitatory and inhibitory interneurons in the dorsal spinal cord. Recent studies have identified a number of postmitotic factors that control the generation of these sensory interneurons. We show that Gsh1/2 and Ascl1 (Mash1), which are expressed in sensory interneuron progenitors, control the choice between excitatory and inhibitory cell fates in the developing mouse spinal cord. During the early phase of neurogenesis, Gsh1/2 and Ascl1 coordinately regulate the expression of Tlx3, which is a critical postmitotic determinant for dorsal glutamatergic sensory interneurons. However, at later developmental times, Ascl1 controls the expression of Ptf1a in dIL(A) progenitors to promote inhibitory neuron differentiation while at the same time upregulating Notch signaling to ensure the proper generation of dIL(B) excitatory neurons. We propose that this switch in Ascl1 function enables the cogeneration of inhibitory and excitatory sensory interneurons from a common pool of dorsal progenitors.

During sleep and awake rest, the neocortex generates large-scale slow-wave activity. Here we report that the claustrum, a poorly understood subcortical neural structure, coordinates neocortical slow-wave generation. We established a transgenic mouse line allowing genetic and electrophysiological interrogation of a subpopulation of claustral glutamatergic neurons.